Elixir v1.2.6 Stream
Module for creating and composing streams.
Streams are composable, lazy enumerables. Any enumerable that generates
items one by one during enumeration is called a stream. For example,
Elixir’s Range is a stream:
iex> range = 1..5
1..5
iex> Enum.map range, &(&1 * 2)
[2, 4, 6, 8, 10]In the example above, as we mapped over the range, the elements being
enumerated were created one by one, during enumeration. The Stream
module allows us to map the range, without triggering its enumeration:
iex> range = 1..3
iex> stream = Stream.map(range, &(&1 * 2))
iex> Enum.map(stream, &(&1 + 1))
[3, 5, 7]Notice we started with a range and then we created a stream that is
meant to multiply each item in the range by 2. At this point, no
computation was done. Only when Enum.map/2 is called we actually
enumerate over each item in the range, multiplying it by 2 and adding 1.
We say the functions in Stream are lazy and the functions in Enum
are eager.
Due to their laziness, streams are useful when working with large
(or even infinite) collections. When chaining many operations with Enum,
intermediate lists are created, while Stream creates a recipe of
computations that are executed at a later moment. Let’s see another
example:
1..3
|> Enum.map(&IO.inspect(&1))
|> Enum.map(&(&1 * 2))
|> Enum.map(&IO.inspect(&1))
1
2
3
2
4
6
#=> [2, 4, 6]Notice that we first printed each item in the list, then multiplied each element by 2 and finally printed each new value. In this example, the list was enumerated three times. Let’s see an example with streams:
stream = 1..3
|> Stream.map(&IO.inspect(&1))
|> Stream.map(&(&1 * 2))
|> Stream.map(&IO.inspect(&1))
Enum.to_list(stream)
1
2
2
4
3
6
#=> [2, 4, 6]Although the end result is the same, the order in which the items were printed changed! With streams, we print the first item and then print its double. In this example, the list was enumerated just once!
That’s what we meant when we said earlier that streams are composable,
lazy enumerables. Notice we could call Stream.map/2 multiple times,
effectively composing the streams and keeping them lazy. The computations
are only performed when you call a function from the Enum module.
Creating Streams
There are many functions in Elixir’s standard library that return streams, some examples are:
IO.stream/2- streams input lines, one by oneURI.query_decoder/1- decodes a query string, pair by pair
This module also provides many convenience functions for creating streams,
like Stream.cycle/1, Stream.unfold/2, Stream.resource/3 and more.
Note the functions in this module are guaranteed to return enumerables. Since enumerables can have different shapes (structs, anonymous functions, and so on), the functions in this module may return any of those shapes and that this may change at any time. For example, a function that today returns an anonymous function may return a struct in future releases.
Summary
Functions
Shortcut to chunk(enum, n, n)
Streams the enumerable in chunks, containing n items each, where
each new chunk starts step elements into the enumerable
Chunks the enum by buffering elements for which fun returns
the same value and only emit them when fun returns a new value
or the enum finishes
Creates a stream that enumerates each enumerable in an enumerable
Creates a stream that enumerates the first argument, followed by the second
Creates a stream that cycles through the given enumerable, infinitely
Creates a stream that only emits elements if they are different from the last emitted element
Creates a stream that only emits elements if the result of calling fun on the element is
different from the (stored) result of calling fun on the last emitted element
Lazily drops the next n items from the enumerable
Lazily drops elements of the enumerable while the given
function returns true
Executes the given function for each item
Creates a stream that filters elements according to the given function on enumeration
Creates a stream that filters and then maps elements according to given functions
Creates a stream that will apply the given function on enumeration and flatten the result
Creates a stream that emits a value after the given period n in milliseconds
Injects the stream values into the given collectable as a side-effect
Emits a sequence of values, starting with start_value. Successive
values are generated by calling next_fun on the previous value
Creates a stream that will apply the given function on enumeration
Creates a stream that will reject elements according to the given function on enumeration
Returns a stream generated by calling generator_fun repeatedly
Emits a sequence of values for the given resource
Runs the given stream
Creates a stream that applies the given function to each element, emits the result and uses the same result as the accumulator for the next computation
Creates a stream that applies the given function to each
element, emits the result and uses the same result as the accumulator
for the next computation. Uses the given acc as the starting value
Lazily takes the next count items from the enumerable and stops
enumeration
Creates a stream that takes every nth item from the enumerable
Lazily takes elements of the enumerable while the given
function returns true
Creates a stream that emits a single value after n milliseconds
Transforms an existing stream
Transforms an existing stream with function-based start and finish
Emits a sequence of values for the given accumulator
Creates a stream that only emits elements if they are unique
Creates a stream where each item in the enumerable will be wrapped in a tuple alongside its index
Zips two collections together, lazily
Types
Functions
Shortcut to chunk(enum, n, n).
chunk(Enumerable.t, pos_integer, pos_integer, Enumerable.t | nil) :: Enumerable.t
Streams the enumerable in chunks, containing n items each, where
each new chunk starts step elements into the enumerable.
step is optional and, if not passed, defaults to n, i.e.
chunks do not overlap. If the final chunk does not have n
elements to fill the chunk, elements are taken as necessary
from pad if it was passed. If pad is passed and does not
have enough elements to fill the chunk, then the chunk is
returned anyway with less than n elements. If pad is not
passed at all or is nil, then the partial chunk is discarded
from the result.
Examples
iex> Stream.chunk([1, 2, 3, 4, 5, 6], 2) |> Enum.to_list
[[1, 2], [3, 4], [5, 6]]
iex> Stream.chunk([1, 2, 3, 4, 5, 6], 3, 2) |> Enum.to_list
[[1, 2, 3], [3, 4, 5]]
iex> Stream.chunk([1, 2, 3, 4, 5, 6], 3, 2, [7]) |> Enum.to_list
[[1, 2, 3], [3, 4, 5], [5, 6, 7]]
iex> Stream.chunk([1, 2, 3, 4, 5, 6], 3, 3, []) |> Enum.to_list
[[1, 2, 3], [4, 5, 6]]Chunks the enum by buffering elements for which fun returns
the same value and only emit them when fun returns a new value
or the enum finishes.
Examples
iex> stream = Stream.chunk_by([1, 2, 2, 3, 4, 4, 6, 7, 7], &(rem(&1, 2) == 1))
iex> Enum.to_list(stream)
[[1], [2, 2], [3], [4, 4, 6], [7, 7]]Creates a stream that enumerates each enumerable in an enumerable.
Examples
iex> stream = Stream.concat([1..3, 4..6, 7..9])
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5, 6, 7, 8, 9]Creates a stream that enumerates the first argument, followed by the second.
Examples
iex> stream = Stream.concat(1..3, 4..6)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5, 6]
iex> stream1 = Stream.cycle([1, 2, 3])
iex> stream2 = Stream.cycle([4, 5, 6])
iex> stream = Stream.concat(stream1, stream2)
iex> Enum.take(stream, 6)
[1, 2, 3, 1, 2, 3]Creates a stream that cycles through the given enumerable, infinitely.
Examples
iex> stream = Stream.cycle([1, 2, 3])
iex> Enum.take(stream, 5)
[1, 2, 3, 1, 2]Creates a stream that only emits elements if they are different from the last emitted element.
This function only ever needs to store the last emitted element.
Elements are compared using ===.
Examples
iex> Stream.dedup([1, 2, 3, 3, 2, 1]) |> Enum.to_list
[1, 2, 3, 2, 1]Creates a stream that only emits elements if the result of calling fun on the element is
different from the (stored) result of calling fun on the last emitted element.
Examples
iex> Stream.dedup_by([{1, :x}, {2, :y}, {2, :z}, {1, :x}], fn {x, _} -> x end) |> Enum.to_list
[{1, :x}, {2, :y}, {1, :x}]Lazily drops the next n items from the enumerable.
If a negative n is given, it will drop the last n items from
the collection. Note that the mechanism by which this is implemented
will delay the emission of any item until n additional items have
been emitted by the enum.
Examples
iex> stream = Stream.drop(1..10, 5)
iex> Enum.to_list(stream)
[6, 7, 8, 9, 10]
iex> stream = Stream.drop(1..10, -5)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]Lazily drops elements of the enumerable while the given
function returns true.
Examples
iex> stream = Stream.drop_while(1..10, &(&1 <= 5))
iex> Enum.to_list(stream)
[6, 7, 8, 9, 10]Executes the given function for each item.
Useful for adding side effects (like printing) to a stream.
Examples
iex> stream = Stream.each([1, 2, 3], fn(x) -> send self, x end)
iex> Enum.to_list(stream)
iex> receive do: (x when is_integer(x) -> x)
1
iex> receive do: (x when is_integer(x) -> x)
2
iex> receive do: (x when is_integer(x) -> x)
3Creates a stream that filters elements according to the given function on enumeration.
Examples
iex> stream = Stream.filter([1, 2, 3], fn(x) -> rem(x, 2) == 0 end)
iex> Enum.to_list(stream)
[2]filter_map(Enumerable.t, (element -> as_boolean(term)), (element -> any)) :: Enumerable.t
Creates a stream that filters and then maps elements according to given functions.
Exists for symmetry with Enum.filter_map/3.
Examples
iex> stream = Stream.filter_map(1..6, fn(x) -> rem(x, 2) == 0 end, &(&1 * 2))
iex> Enum.to_list(stream)
[4, 8, 12]Creates a stream that will apply the given function on enumeration and flatten the result.
Examples
iex> stream = Stream.flat_map([1, 2, 3], fn(x) -> [x, x * 2] end)
iex> Enum.to_list(stream)
[1, 2, 2, 4, 3, 6]Creates a stream that emits a value after the given period n in milliseconds.
The values emitted are an increasing counter starting at 0.
Examples
iex> Stream.interval(10) |> Enum.take(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]Injects the stream values into the given collectable as a side-effect.
This function is often used with run/1 since any evaluation
is delayed until the stream is executed. See run/1 for an example.
Emits a sequence of values, starting with start_value. Successive
values are generated by calling next_fun on the previous value.
Examples
iex> Stream.iterate(0, &(&1+1)) |> Enum.take(5)
[0, 1, 2, 3, 4]Creates a stream that will apply the given function on enumeration.
Examples
iex> stream = Stream.map([1, 2, 3], fn(x) -> x * 2 end)
iex> Enum.to_list(stream)
[2, 4, 6]Creates a stream that will reject elements according to the given function on enumeration.
Examples
iex> stream = Stream.reject([1, 2, 3], fn(x) -> rem(x, 2) == 0 end)
iex> Enum.to_list(stream)
[1, 3]Returns a stream generated by calling generator_fun repeatedly.
Examples
# Although not necessary, let's seed the random algorithm
iex> :rand.seed(:exsplus, {1, 2, 3})
iex> Stream.repeatedly(&:rand.uniform/0) |> Enum.take(3)
[0.40502929729990744, 0.45336720247823126, 0.04094511692041057]Emits a sequence of values for the given resource.
Similar to transform/2 but the initial accumulated value is
computed lazily via start_fun and executes an after_fun at
the end of enumeration (both in cases of success and failure).
Successive values are generated by calling next_fun with the
previous accumulator (the initial value being the result returned
by start_fun) and it must return a tuple containing a list
of items to be emitted and the next accumulator. The enumeration
finishes if it returns {:halt, acc}.
As the name says, this function is useful to stream values from resources.
Examples
Stream.resource(fn -> File.open!("sample") end,
fn file ->
case IO.read(file, :line) do
data when is_binary(data) -> {[data], file}
_ -> {:halt, file}
end
end,
fn file -> File.close(file) end)Runs the given stream.
This is useful when a stream needs to be run, for side effects, and there is no interest in its return result.
Examples
Open up a file, replace all # by % and stream to another file
without loading the whole file in memory:
stream = File.stream!("code")
|> Stream.map(&String.replace(&1, "#", "%"))
|> Stream.into(File.stream!("new"))
|> Stream.runNo computation will be done until we call one of the Enum functions
or Stream.run/1.
Creates a stream that applies the given function to each element, emits the result and uses the same result as the accumulator for the next computation.
Examples
iex> stream = Stream.scan(1..5, &(&1 + &2))
iex> Enum.to_list(stream)
[1, 3, 6, 10, 15]Creates a stream that applies the given function to each
element, emits the result and uses the same result as the accumulator
for the next computation. Uses the given acc as the starting value.
Examples
iex> stream = Stream.scan(1..5, 0, &(&1 + &2))
iex> Enum.to_list(stream)
[1, 3, 6, 10, 15]Lazily takes the next count items from the enumerable and stops
enumeration.
If a negative count is given, the last count values will be taken.
For such, the collection is fully enumerated keeping up to 2 * count
elements in memory. Once the end of the collection is reached,
the last count elements will be executed. Therefore, using
a negative count on an infinite collection will never return.
Examples
iex> stream = Stream.take(1..100, 5)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]
iex> stream = Stream.take(1..100, -5)
iex> Enum.to_list(stream)
[96, 97, 98, 99, 100]
iex> stream = Stream.cycle([1, 2, 3]) |> Stream.take(5)
iex> Enum.to_list(stream)
[1, 2, 3, 1, 2]Creates a stream that takes every nth item from the enumerable.
The first item is always included, unless nth is 0.
nth must be a non-negative integer, or FunctionClauseError will be thrown.
Examples
iex> stream = Stream.take_every(1..10, 2)
iex> Enum.to_list(stream)
[1, 3, 5, 7, 9]
iex> stream = Stream.take_every([1, 2, 3, 4, 5], 1)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]
iex> stream = Stream.take_every(1..1000, 0)
iex> Enum.to_list(stream)
[]Lazily takes elements of the enumerable while the given
function returns true.
Examples
iex> stream = Stream.take_while(1..100, &(&1 <= 5))
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]Creates a stream that emits a single value after n milliseconds.
The value emitted is 0.
Examples
iex> Stream.timer(10) |> Enum.to_list
[0]transform(Enumerable.t, acc, fun) :: Enumerable.t when fun: (element, acc -> {Enumerable.t, acc} | {:halt, acc}), acc: any
Transforms an existing stream.
It expects an accumulator and a function that receives each stream item
and an accumulator, and must return a tuple containing a new stream
(often a list) with the new accumulator or a tuple with :halt as first
element and the accumulator as second.
Note: this function is similar to Enum.flat_map_reduce/3 except the
latter returns both the flat list and accumulator, while this one returns
only the stream.
Examples
Stream.transform/3 is useful as it can be used as the basis to implement
many of the functions defined in this module. For example, we can implement
Stream.take(enum, n) as follows:
iex> enum = 1..100
iex> n = 3
iex> stream = Stream.transform(enum, 0, fn i, acc ->
...> if acc < n, do: {[i], acc + 1}, else: {:halt, acc}
...> end)
iex> Enum.to_list(stream)
[1, 2, 3]transform(Enumerable.t, (() -> acc), fun, (acc -> term)) :: Enumerable.t when fun: (element, acc -> {Enumerable.t, acc} | {:halt, acc}), acc: any
Transforms an existing stream with function-based start and finish.
The accumulator is only calculated when transformation starts. It also allows an after function to be given which is invoked when the stream halts or completes.
This function can be seen as a combination of Stream.resource/3 with
Stream.transform/3.
Emits a sequence of values for the given accumulator.
Successive values are generated by calling next_fun with the previous
accumulator and it must return a tuple with the current value and next
accumulator. The enumeration finishes if it returns nil.
Examples
iex> Stream.unfold(5, fn 0 -> nil; n -> {n, n-1} end) |> Enum.to_list()
[5, 4, 3, 2, 1]Creates a stream that only emits elements if they are unique.
Keep in mind that, in order to know if an element is unique or not, this function needs to store all unique values emitted by the stream. Therefore, if the stream is infinite, the number of items stored will grow infinitely, never being garbage collected.
Examples
iex> Stream.uniq([1, 2, 3, 3, 2, 1]) |> Enum.to_list
[1, 2, 3]
iex> Stream.uniq([{1, :x}, {2, :y}, {2, :z}, {1, :x}], fn {x, _} -> x end) |> Enum.to_list
[{1, :x}, {2, :y}]Creates a stream where each item in the enumerable will be wrapped in a tuple alongside its index.
Examples
iex> stream = Stream.with_index([1, 2, 3])
iex> Enum.to_list(stream)
[{1, 0}, {2, 1}, {3, 2}]
iex> stream = Stream.with_index([1, 2, 3], 3)
iex> Enum.to_list(stream)
[{1, 3}, {2, 4}, {3, 5}]